Since the advent of high power lasers there has been an increased emphasis on the protection of low light level sensors (including human eyes). It is well known that optical sensors can be damaged by exposure to intense laser radiation of both pulsed and CW varieties. Protectors or optical limiters may be of two general types, active or passive. Active optical limiters require a predetermination of the presence of threat radiation and then must provide an external stimulus (such as an applied electric field) to operate the limiter. Active devices are usually complex (tunable filters for example) and are unable to respond to short pulses or bursts of harmful radiation. Passive protectors are preferred since the threat radiation itself triggers the desired protective response.
An ideal protector must fulfill various requirements: it must not degrade or attenuate desired radiation (low insertion loss), it must provide complete blocking of radiation above some predetermined threshold (harmful to the sensor or eyes), it must be sensitive over a sufficiently wide wavelength range to block all undesired radiation, it must have a wide field-of-view, it must (in many cases) be fast acting (for pulsed lasers), and it must be capable of simultaneously blocking a multiplicity of intense laser wavelengths emanating from a single source.
There has been considerable interest in protecting all electro-optical sensor systems including; for example, infrared detectors; against high-power infrared laser threats such as line-tunable carbon dioxide (CO.sub.2) lasers. These IR detectors are used in night vision devices by the military and are vulnerable to both damage and jamming (overload) in the presence of high-energy laser countermeasures. The output of such a laser could be at any nominal frequency within the response band of the detector in a night vision device. In particular, sensitive infrared optical detectors used in systems with high optical gain (focusing) such as FLIRs can be rendered useless and even destroyed by accidental or deliberate illumination with intense far infrared (FIR) sources, particularly CO.sub.2 lasers. The object of the present invention is, therefor, to passively protect optical sensors, detectors, the eye, as well as night vision devices such as FLIR's.